Simple boronic acids are inhibitors of serine proteases. For example, Koehler et al. Biochemistry 10: 2477 (1971) reports that 2-phenylethane boronic acid inhibits chymotrypsin at millimolar levels. The synthesis of boronic acid analogs of N-acyl-a-amino acids has yielded more effective inhibitors. Ac-boroPhe-OH, R-1-acetamido-2-phenylethane boronic acid, inhibits chymotrypsin with a K.sub.i of 4 .mu.M Matteson et al. J. Am. Chem. Soc. 103: 5241 (1981). More recently, Shenvi, U.S. Pat No. 4,537,773 (1985) disclosed that boronic acid analogs of .alpha.-amino acids, containing a free amino group, were effective inhibitors of aminopeptidases. Shenvi, U.S. Pat. No. 4,499,082 (1985) discloses that peptides containing an .alpha.-amino boronic acid with a neutral side chain were more effective inhibitors of serine proteases exceeding inhibitors disclosed earlier by as much as 3 orders of magnitude in potency. The chemistry of .alpha.-aminoboronic acids was further expanded to the synthesis of peptide analogs containing boronic acid with positive charged side chains, boroLysine, boroArginine, boroOrnithine, and isothiouronium analogs. This is disclosed in Kettner, et al. EPA 0,293,881, published Dec. 7, 1988.
Much progress has been made in the synthesis of boronic acid and corresponding peptides with the boronic acid protected as an ester. However, a convenient method of removal of the ester protecting group is lacking. Matteson (1981) infra, reports the destructive removal of pinanediol group by treatment with anhydrous BCl.sub.3. Kettner and Shenvi J. Biol. Chem. 259: 15106 (1984) describe the removal of the pinacol protecting group by converting the boronic pinacol esters to the thermodynamically more stable, diethanolamine ester by transesterification and then hydrolysis by treatment with aqueous acid or with a cation exchange resin. This method is not applicable for removal of pinanediol ester due to its greater stability. Matteson Chem. Rev. 89: 1535 (1989) describes the removal of the pinanediol group in situ by incubations in borate buffer. It should be noted that the pinanediol ester is preferred in synthesis due to it ability to direct stereochemistry at the .alpha.-carbon of boronic acid and its stability to chemical manipulations. The pinanediol protecting group was used almost exclusively in the preparation of boroArginine peptides, shown in EPA 0,293,881. In one example, partial hydrolysis of the pinanediol ester was obtained by binding Ac-(D)Phe-Pro-boroArg-C.sub.10 H.sub.16 to a cation exchange resin and washing extensively with aqueous acetic acid followed by elution with HCl. This reaction is slow, it requires recovery of product by evaporation of large volumes of water and separation of the free boronic acid from the ester . Removal of the pinanediol by treatment with BCl.sub.3 as the final step in synthesis was considered to be the only practical method.